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| United States Patent |
5,135,713
|
|
Rioja
, et al.
|
*
August 4, 1992
|
Aluminum-lithium alloys having high zinc
Abstract
An aluminum base alloy suitable for forming into a wrought product having
improved combinations of strength and fracture toughness is disclosed. The
product is comprised of 0.2 to 3.0 wt. % Li, 0.1 to 3 wt. % Mg, 0.2 to 3
wt. % Cu, 5.1 to 12 wt. % Zn, 0.5 wt. % max. Fe, 0.5 wt. % max. Si, at
least one of the elements selected from the group Cr, V, Hf, Mn, Ti, Ag,
In and Zr, with Cr, V, Ti and Zr in the range of 0.01 to 0.2 wt. %; Hf and
Mn up to 0.6 wt. % each, Ag in the range of 0.05 to 1 wt. % and in the
range of 0.01 to 0.5 wt. %, the balance aluminum and incidental
impurities.
| Inventors:
|
Rioja; Roberto J. (Lower Burrell, PA);
Staley; James T. (Murrysville, PA)
|
| Assignee:
|
Aluminum Company of America (Pittsburgh, PA)
|
| [*] Notice: |
The portion of the term of this patent subsequent to March 10, 2004
has been disclaimed. |
| Appl. No.:
|
588410 |
| Filed:
|
September 26, 1990 |
| Current U.S. Class: |
420/532; 148/417; 148/418; 148/439; 420/535; 420/539 |
| Intern'l Class: |
C22C 021/06 |
| Field of Search: |
420/532,534,535,539,541,542,543
148/439,417,418
|
References Cited
U.S. Patent Documents
| 4869870 | Sep., 1989 | Rioja et al. | 420/532.
|
| 4961792 | Oct., 1990 | Rioja et al. | 148/417.
|
| Foreign Patent Documents |
| 0057049 | Mar., 1969 | PL | 420/541.
|
Other References
Baba Yoshios: "Effects of Additional Elements and Heat Treatments on the
Fracture Characteristics of Al-Zn-Mg Alloy,", J. Japan Inst. Metals
(Trans. JIM): 1970, vol. 11, No. 6, pp. 404-410.
|
Primary Examiner: Dean; B.
Assistant Examiner: Koehler; Robert R.
Attorney, Agent or Firm: Alexander; Andrew
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This is a continuation-in-part of U.S. Pat. application Ser. No. 149,802,
filed Jan. 28, 1988 and U.S. Pat. application Ser. No. 172,506, filed Mar.
24, 1988, now U.S. Pat. No. 4,961,792, issued Oct. 9, 1990, is a
continuation-in-part of U.S. Pat. application Ser. No. 685,731, filed Dec.
24, 1984, now U.S. Pat. No. 4,797,165, issued Jan. 10, 1989, which is a
continuation-in-part of U.S. Serial No. 594,344, filed March 29, 1984, now
U.S. Patent 4,648,913, issued March 10, 1987.
Claims
What is claimed is:
1. An aluminum base alloy suitable for forming into a wrought product
having improved combinations of strength and fracture toughness, the
product comprised of 0.6 to 3.0 wt.% Li, 0.5 to 3 wt.% Mg, 0.2 to 3 wt.%
Cu, 5.1 to 12 wt.% Zn, 0.5 wt.% max. Fe, 0.5 wt.% max. Si, at least one of
the elements selected from the group Cr, V, Hf, Mn, Ti, Ag, In and Zr,
with Cr, V, Ti and Zr in the range of 0.01 to 1 wt.%; Hf and Mn up to 2
wt.% each, Ag in the range of 0.05 to 1 wt.% and In in the range of 0.01
to 0.5 wt.%, the balance aluminum and incidental impurities.
2. The alloy in accordance with claim 1 wherein Cu is in the range of 0.05
to 2.44.
3. The alloy in accordance with claim 1 wherein Cu is in the range of 0.2
to 2.4.
4. The alloy in accordance with claim 1 wherein Cu is in the range of 0.2
to 2.
5. The alloy in accordance with claim 1 wherein Mg is in the range of 0.5
to 2.5.
6. The alloy in accordance with claim 1 wherein Zn is in the range of 5.2
to 11.
7. The alloy in accordance with claim 1 wherein Zn is in the range of 5.5
to 11.
8. The alloy in accordance with claim 1 wherein Zn is in the range of 7.1
to 10.
9. The alloy in accordance with claim 1 wherein Zn is in the range of 7.2
to 10.
10. The alloy in accordance with claim 1 wherein Zn is in the range of 7.5
to 10.
11. The alloy in accordance with claim 1 wherein Zr is in the range of 0.01
to 0.5.
12. The alloy in accordance with claim 1 wherein Mn is 1 wt.% max.
13. An aluminum base alloy suitable for forming into a wrought product
having improved combinations of strength and fracture toughness, the
product comprised of 0.2 to 3.0 wt.% Li, 0.5 to 2.5 wt.% Mg, 0.2 to 2.4
wt.% Cu, 7.2 to 11 wt.% Zn, 0.5 wt.% max. Fe, 0.5 wt.% max. Si, at least
one of the elements selected from the group Cr, V, Hf, Mh, Ti, Ag, In and
Zr, with Cr, V, Ti and Zr in the range of 0.01 to 0.5 wt.%; Hf and Mn up
to 1 wt.% each, Ag in the range of 0.05 to 1 and In in the range of 0.01
to 0.5, the balance aluminum and incidental impurities.
14. An aluminum base alloy suitable for forming into a wrought product
having improved combinations of strength and fracture toughness, the
product comprised of 0.2 to 3.0 wt.% Li, 0.1 to 3 wt.% Mg, 0.2 to 2 wt.%
Cu, 7.5 to 10 wt.% Zn, 0.5 wt.% max. Fe, 0.5 wt.% max. Si, at least one of
the elements selected from the group Cr, V, Hf, Mn, Ti, Ag, In and Zr,
with Cr, V, Ti and Zr in the range of 0.01 to 0.2 wt.%; Hf and Mn up to
0.6 wt.% each, Ag in the range of 0.05 to 1 and In in the range of 0.01 to
0.5, the balance aluminum and incidental impurities.
Description
BACKGROUND OF THE INVENTION
This invention relates to aluminum base alloy products, and more
particularly, it relates to improved lithium containing aluminum base
alloy products having improved corrosion resistance and a method of
producing the same.
In the aircraft industry, it has been generally recognized that one of the
most effective ways to reduce the weight of an aircraft is to reduce the
density of aluminum alloys used in the aircraft construction. For purposes
of reducing the alloy density, lithium additions have been made. However,
the addition of lithium to aluminum alloys is not without problems. For
example, the addition of lithium to aluminum alloys often results in a
decrease in ductility and fracture toughness. Where the use is in aircraft
parts, it is imperative that the lithium containing alloy have both
improved fracture toughness and strength properties.
It will be appreciated that both high strength and high fracture toughness
appear to be quite difficult to obtain when viewed in light of
conventional alloys such as AA (Aluminum Association) 2024-T3X and 7050-TX
normally used in aircraft applications. For example, a paper by J. T.
Staley entitled "Microstructure and Toughness of High-Strength Aluminum
Alloys", Properties Related to Fracture Toughness, ASTM STP605, American
Society for Testing and Materials, 1976, pp. 71-103, shows generally that
for AA2024 sheet, toughness decreases as strength increases. Also, in the
same paper, it will be observed that the same is true of AA7050 plate.
More desirable alloys would permit increased strength with only minimal or
no decrease in toughness or would permit processing steps wherein the
toughness was controlled as the strength was increased in order to provide
a more desirable combination of strength and toughness. Additionally, in
more desirable alloys, the combination of strength and toughness would be
attainable in an aluminum-lithium alloy having density reductions in the
order of 5 to 15%. Such alloys would find widespread use in the aerospace
industry where low weight and high strength and toughness translate to
high fuel savings. Thus, it will be appreciated that obtaining qualities
such as high strength at little or no sacrifice in toughness, or where
toughness can be controlled as the strength is increased would result in a
remarkably unique aluminum-lithium alloy product.
The present invention provides an improved lithium containing aluminum base
alloy product which can be processed to improve strength characteristics
while retaining high toughness properties or which can be processed to
provide a desired strength at a controlled level of toughness.
SUMMARY OF THE INVENTION
A principal object of this invention is to provide a lithium containing
aluminum base alloy product.
Another object of this invention is to provide an improved aluminum-lithium
alloy wrought product having improved strength and toughness
characteristics.
Yet another object of this invention is to provide an Aluminum Association
7000-type aluminum alloy containing lithium having improved properties.
And yet another object of this invention includes a method of providing a
wrought aluminum-lithium alloy product having improved corrosion
resistance and working the product after solution heat treating to
increase strength properties without substantially impairing its fracture
toughness.
And yet a further object of this invention is to provide a method of
increasing the strength of a wrought aluminum-lithium alloy product after
solution heat treating without substantially decreasing fracture
toughness.
These and other objects will become apparent from the specification,
drawings and claims appended hereto.
An aluminum base alloy suitable for forming into a wrought product having
improved combinations of strength and fracture toughness is disclosed. The
product is comprised of 0.2 to 3.0 wt.% Li, 0.5 to 3 wt.% Mg, 0.2 to 3
wt.% Cu, 5.1 to 12 wt.% Zn, 0.5 wt.% max. Fe, 0.5 wt.% max. Si, at least
one of the elements selected from the group Cr, V, Hf, Mn, Ti, Ag, In and
Zr, with Cr, V, Ti and Zr in the range of 0.01 to 0.2 wt.%; Hf and Mn up
to 0.6 wt.% each, Ag in the range of 0.05 to 1 wt.% and In in the range of
0.01 to 0.5 wt.%, the balance aluminum and incidental impurities.
The product is capable of having imparted thereto a working effect
equivalent to stretching so that the product has combinations of improved
strength and fracture toughness after aging. In the method of making an
aluminum base alloy product having improved combinations of strength,
fracture toughness and corrosion resistance, a body of a lithium
containing aluminum base alloy is provided and may be worked to produce a
wrought aluminum product. The wrought product may be first solution heat
treated and then stretched or otherwise worked amount equivalent to
stretching. The degree of working as by stretching, for example, is
normally greater than that used for relief of residual internal quenching
stresses.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing tensile yield strength versus aging time at
250.degree. F. and 300.degree. F.
FIG. 2 is a graph plotting typical fracture toughness versus tensile yield
strength for alloy products made in accordance with the invention, as well
as for 7075-T651 and 7150-T651 alloy products.
FIG. 3 is a bar graph plotting the specific strength of an alloy product of
the invention versus the specific strength of alloy products made from
7075-T651 and 7150-T651 alloys.
FIG. 4 shows part of an Al-Zn-Mg-Cu phase diagram for selecting the
composition of the base alloy prior to Li additions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The alloy of the present invention can contain 0.2 to 5.0 wt.% Li, 0 to 5.0
wt.% Mg, up to 5.0 wt.% Cu, 0 to 1.0 wt.% Zr, preferably up to 0.5 wt.%
Zr, 0 to 2.0 wt.% Mn, 0.05 to 12.0 wt.% Zn, 0.5 wt.% max. Fe, 0.5 wt.%
max. Si, the balance aluminum and incidental impurities. The impurities
are preferably limited to about 0.05 wt.% each, and the combination of
impurities preferably should not exceed 0.15 wt.%. Within these limits, it
is preferred that the sum total of all impurities does not exceed 0.35
wt.%.
An alloy in accordance with the present invention can contain 0.2 to 5.0
wt.% Li, at least 2.45 wt.% Cu, 0.05 to 5.0 wt.% Mg, 0.05 to 0.16 wt.% Zr,
0.05 to 12.0 wt.% Zn, the balance aluminum and impurities as specified
above. A further alloy composition would contain 0.5 to 2.0 wt.% Li, 2.55
to 2.90 wt.% Cu, 0.2 to 2.5 wt.% Mg, 0.2 to 11.0 wt.% Zn, 0.08 to 0.12
wt.% Zr, 0 to 1.0 wt.% Mn and max. 0.1 wt.% of each of Fe and Si. In a
preferred typical alloy, Zn may be in the range of 0.2 to 2.0 and Mg 0.2
to 2.0 wt.%.
In another aspect of the invention, the alloy can be comprised of 0.2 to
3.0 wt.% Li, 0.5 to 3 wt.% Mg, 0.05 to 3 wt.% Cu, 5.1 to 12 wt.% Zn, 0.5
wt.% max. Fe, 0.5 wt.% max. Si, at least one of the elements selected from
the group Cr, V, Hf, Mn, Ti, Ag, In and Zr, with Cr, V, Ti and Zr in the
range of 0.01 to 0.2 wt.%; Hf and Mn up to 0.6 wt.% each, Ag in the range
of 0.05 to 1 wt.% and In in the range of 0.01 to 0.5 wt.%, the balance
aluminum and incidental impurities. Copper can range from 0.2 to 2.44 wt.%
with suitable amounts being 0.2 to 2.0 wt.%.
In the present invention, lithium is very important not only because it
permits a significant decrease in density but also because it improves
tensile and yield strengths markedly as well as improving elastic modulus.
Additionally, the presence of lithium improves fatigue resistance. Most
significantly though, the presence of lithium in combination with other
controlled amounts of alloying elements permits aluminum alloy products
which can be worked to provide unique combinations of strength and
fracture toughness while maintaining meaningful reductions in density. It
will be appreciated that less than 0.5 wt.% Li does not provide for
significant reductions in the density of the alloy. It is not presently
expected that higher levels of lithium would improve the combination of
toughness and strength of the alloy product.
It must be recognized that to obtain a high level of corrosion resistance
in addition to the unique combinations of strength and fracture toughness
as well as reductions in density requires careful selection of all the
alloying elements. For example, for every 1 wt.% Li added, the density of
the alloy is decreased about 2.4%. Thus, if density is the only
consideration, then the amount of Li would be maximized. However, if it is
desired to increase toughness at a given strength level, then Cu should be
added. However, for every 1 wt.% Cu added to the alloy, the density is
increased by 0.87% and resistance to corrosion. Likewise, for every 1 wt.%
Mn added, the density is increased about 0.85%. Thus, care must be taken
to avoid losing the benefits of lithium by the addition of alloying
elements such as Cu and Mn, for example. Accordingly, while lithium is the
most important element for saving weight, the other elements are important
in order to provide the proper levels of strength, fracture toughness,
corrosion and stress corrosion cracking resistance.
With respect to copper, particularly in the ranges set forth hereinabove
for use in accordance with the present invention, its presence enhances
the properties of the alloy product by reducing the loss in fracture
toughness at higher strength levels. That is, as compared to lithium, for
example, in the present invention copper has the capability of providing
higher combinations of toughness and strength. For example, if more
additions of lithium were used to increase strength without copper, the
decrease in toughness would be greater than if copper additions were used
to increase strength. Thus, in the present invention when selecting an
alloy, it is important in making the selection to balance both the
toughness and strength desired, since both elements work together to
provide toughness and strength uniquely in accordance with the present
invention. It is important that the ranges referred to hereinabove, be
adhered to, particularly with respect to the upper limits of copper, since
excessive amounts can lead to the undesirable formation of intermetallics
which can interfere with fracture toughness. In addition, higher levels of
copper can result in diminished resistance to corrosion. Typically, copper
should be less than 3.0 wt.%; however, copper can be increased to less
than 5 wt.% and preferably less than 4 wt.% and typically less than 3.5
wt.%. The combination of lithium and copper should not exceed 5.5 wt.%
with lithium being at least 0.5 wt.% with greater amounts of lithium being
preferred. Thus, in accordance with this invention, it has been discovered
that adhering to the ranges set forth above for copper, fracture
toughness, strength, corrosion and stress corrosion cracking can be
maximized.
Magnesium is added or provided in this class of aluminum alloys mainly for
purposes of increasing strength although it does decrease density slightly
and is advantageous from that standpoint. It is important to adhere to the
upper limits set forth for magnesium because excess magnesium can also
lead to interference with fracture toughness, particularly through the
formation of undesirable phases at grain boundaries.
Zirconium may be added for grain structure control; however, other elements
which may control grain structure or improve strength can include Cr, V,
Hf, Mn, Sc, Ti, Ag and In, with Cr, V, Mn and Ti being in the range of
0.05 to 0.2 wt.% with Hf and Mn up to typically 0.6 wt.%, Ag in the range
of 0.05 to 1 wt.% and In in the range of 0.01 to 0.5 wt.%. The level of
Zr, Hf or Sc used depends on whether a recrystallized or unrecrystallized
structure is desired. The use of zinc results in increased levels of
strength, particularly in combination with magnesium. However, excessive
amounts of zinc can impair toughness through the formation of
intermetallic phases.
Zinc is important because, in this combination with magnesium, it results
in an improved level of strength. In a preferred aspect, Zn can range from
5.1 to 11 wt.%, preferably 5.2 to 10 wt.% with typical amounts being in
the range of 7.1 to 11 wt.% and suitable amounts being 7.2 or 7.5 to 10
wt.%.
Toughness or fracture toughness as used herein refers to the resistance of
a body, e.g. sheet or plate, to the unstable growth of cracks or other
flaws.
As well as providing the alloy product with controlled amounts of alloying
elements as described hereinabove, it is preferred that the alloy be
prepared according to specific method steps in order to provide the most
desirable characteristics of both strength and fracture toughness. Thus,
the alloy as described herein can be provided as an ingot or billet for
fabrication into a suitable wrought product by casting techniques
currently employed in the art for cast products, with continuous casting
being preferred. Further, the alloy may be roll cast or slab cast to
thicknesses from about 1/4 to 2 or 3 inches or more depending on the end
product desired. It should be noted that the alloy may also be provided in
billet form consolidated from fine particulate such as powdered aluminum
alloy having the compositions in the ranges set forth hereinabove. The
powder or particulate material can be produced by processes such as
atomization, mechanical alloying and melt spinning. The ingot or billet
may be preliminarily worked or shaped to provide suitable stock for
subsequent working operations. Prior to the principal working operation,
the alloy stock is preferably subjected to homogenization, and preferably
at metal temperatures in the range of 860 to 1050.degree. F. for a period
of time of at least one hour to dissolve soluble elements such as Li and
Cu, and to homogenize the internal structure of the metal. A preferred
time period is about 20 hours or more in the homogenization temperature
range. Normally, the heat up and homogenizing treatment does not have to
extend for more than 40 hours; however, longer times are not normally
detrimental. A time of 20 to 40 hours at the homogenization temperature
has been found quite suitable. In addition to dissolving constituent to
promote workability, this homogenization treatment is important in that it
is believed to precipitate the Mn and Zr-bearing dispersoids which help to
control final grain structure.
After the homogenizing treatment, the metal can be rolled or extruded or
otherwise subjected to working operations to produce stock such as sheet,
plate or extrusions or other stock suitable for shaping into the end
product. To produce a sheet or plate-type product, a body of the alloy is
preferably hot rolled to a thickness ranging from 0.1 to 0.25 inch for
sheet and 0.25 to 6.0 inches for plate. For hot rolling purposes, the
temperature should be in the range of 950.degree. F. down to 550.degree.
F. Preferably, the metal temperature initially is in the range of 850 to
750.degree. F.
When the intended use of a plate product is for wing spars where thicker
sections are used, normally operations other than hot rolling are
unnecessary. Where the intended use is wing or body panels requiring a
thinner gauge, further reductions as by cold rolling can be provided. Such
reductions can be to a sheet thickness ranging, for example, from 0.010 to
0.249 inch and usually from 0.030 to 0.25 inch.
After rolling a body of the alloy to the desired thickness, the sheet or
plate or other worked article is subjected to a solution heat treatment to
dissolve soluble elements. The solution heat treatment is preferably
accomplished at a temperature in the range of 850 to 1050.degree. F. and
preferably produces an unrecrystallized grain structure.
Solution heat treatment can be performed in batches or continuously, and
the time for treatment can vary from hours for batch operations down to as
little as a few seconds for continuous operations. Basically, solution
effects can occur fairly rapidly, for instance in as little as 30 to 60
seconds, once the metal has reached a solution temperature of about 850 to
1050.degree. F. However, heating the metal to that temperature can involve
substantial amounts of time depending on the type of operation involved.
In batch treating a sheet product in a production plant, the sheet is
treated in a furnace load and an amount of time can be required to bring
the entire load to solution temperature, and accordingly, solution heat
treating can consume one or more hours, for instance one or two hours or
more in batch solution treating. In continuous treating, the sheet is
passed continuously as a single web through an elongated furnace which
greatly increases the heat-up rate. The continuous approach is favored in
practicing the invention, especially for sheet products, since a
relatively rapid heat up and short dwell time at solution temperature is
obtained. Accordingly, the inventors contemplate solution heat treating in
as little as about 1.0 minute. As a further aid to achieving a short
heat-up time, a furnace temperature or a furnace zone temperature
significantly above the desired metal temperature provides a greater
temperature head useful in reducing heat-up times. However, when an
unrecrystallized microstructure is required, a recovery annealing practice
will be required prior to solution heat treatment.
To further provide for the desired strength and fracture toughness, as well
as corrosion resistance, necessary to the final product and to the
operations in forming that product, the product should be rapidly quenched
to prevent or minimize uncontrolled precipitation of strengthening phases
referred to herein later.
After the alloy product of the present invention has been quenched and
worked, if desired, it may be artificially aged to provide the combination
of fracture toughness and strength which are so highly desired in aircraft
members. This can be accomplished by subjecting the sheet or plate or
shaped product to a temperature in the range of 150 to 400.degree. F. for
a sufficient period of time to further increase the yield strength. Some
compositions of the alloy product are capable of being artificially aged
to a yield strength higher than 100 ksi. However, the useful strengths are
in the range of 50 to 95 ksi and corresponding fracture toughnesses are in
the range of 20 to 75 ksi vin. Preferably, artificial aging is
accomplished by subjecting the alloy product to a temperature in the range
of 200 to 375.degree. F. for a period of at least 30 minutes. A suitable
aging practice contemplate a treatment of about 8 to 48 hours at a
temperature of about 250.degree. F. Further, it will be noted that the
alloy product in accordance with the present invention may be subjected to
any of the typical underaging and overaging treatments well known in the
art, including natural aging. Also, while reference has been made herein
to single aging steps, multiple aging steps, such as two or three aging
steps, are contemplated and stretching or its equivalent working may be
used prior to or even after part of such multiple aging steps.
The product may be subjected to three aging steps, phases or treatments,
although clear lines of demarcation may not exist between each step or
phase. Ramping up to and/or down from given (or target) treatment
temperatures, in itself, can produce precipitation (aging) effects which
can, and often need to be, taken into account by integrating such ramping
conditions, and their precipitation-hardening effects, into the total
aging treatment program. With ramping and its corresponding integration,
the three phases for thermally treating invention alloy according to this
aging practice may be effected in a single, programmable furnace. For
convenience purposes, though, each stage (step or phase) will be more
fully described as a distinct operation hereafter. It is believed that the
first stage serves to precipitation harden the alloy product, the second
(higher temperature) stage then exposes alloy product to one or more
elevated temperatures for increasing its resistance to exfoliation and
stress corrosion cracking (SCC), while the third stage further
precipitation hardens the invention alloy to a very high strength level.
In the first treatment stage, the alloy of the invention is precipitation
hardened to strengthen it, for example, to a point at or near peak
strength (whether underaged or slightly overaged) although less than peak
strength conditions (or underaging) may be desired in some cases. Such
precipitation hardening can be accomplished by heating to one or more
elevated temperatures below about 300.degree. F, preferably between about
200 and 300.degree. F., for a period of time ranging from about 5 or 10
hours to about 40 hours or more. A substantially similar treatment may
occur through gradual ramping to the second (higher temperature) treatment
stage, with or without any hold time at temperatures in said first range.
This first treatment phase can also extend until the alloy achieves up to
about 95% of peak strength (underaged), peak strength itself, or even
until alloy strength runs slightly past peak and back down to about 95% of
peak strength (through overaging). It should be understood, however, that
for some embodiments, relative yield strengths may also increase during
the second treatment phase depending on the extent to which peak yield
strength was approached during the first treatment phase.
Following this first phase of thermal treatment, the alloy of the invention
is preferably subjected to heating at temperatures above about 400.degree.
F., preferably within the range of from about 350 to about 450.degree. F.,
for a few minutes or more (e.g., for 3 or more minutes), preferably more
than three minutes, to about 3 to 20 hours or more. Generally, alloy
exposure temperatures vary inversely with duration such that shorter times
are used at relatively higher temperatures, while longer times are more
appropriate at the lower temperatures, below about 400.degree. F. or so.
When heating alloy products to one or more temperatures for "x" minutes
according to this preferred second treatment phase, it is to be understood
that such treatment embraces heating to any number of temperatures within
said range for a cumulative time "x" above the lowest temperature of said
range. As such, heating for 5 or more minutes within about 350 to
450.degree. F. does not require holding for 5 minutes at each or even any
particular temperature within said range, but rather, that the cumulative
time at all temperatures within 350 to 450.degree. F. is 5 minutes or
more.
The second phase treatment may be carried out by immersing the alloy
products into a substantially hot liquid such as molten salt, hot oil or
even molten metal. A furnace (hot air and/or other gases) may also be used
depending on the size, shape and quantity of the product to be treated. In
the alternative, a fluidized bed-type apparatus may be used, such an
apparatus providing more rapid heating than a hot air furnace but slower,
more uniform heating than a molten salt bath. Fluidized bed heat-ups are
especially advantageous for presenting fewer environmental complications.
Induction heaters may also be used for artificial aging according to the
invention, for instance, in the second phase of this preferred method.
During the third phase of this preferred treatment method, the alloy
product of the invention is subjected to one or more elevated temperatures
up to about 300.degree. F., preferably between about 150 and about
250.degree. F., for about 2 to 30 hours or more. With such treatment, the
invention alloy is able to achieve significantly higher strength levels
than those attained by 7075 and other 7XXX aluminum alloy product
counterparts. When aged to achieve corrosion properties comparable to
those of T6-aged products, for example, having EXCO corrosion ratings of
"EB" or better, the alloy products of the invention produces minimum yield
strengths (compression and/or tension) at least about 15% greater than the
minimum strengths for a similarly-sized, shaped and formed 7X50 alloy
product aged to a T76 temper, and at least about 15% greater relative
strength than a 7X50-T6 product.
Relative strength values for artificially aged alloy products of the
invention will vary to some extent depending on their size, shape and
method of manufacture. For example, improved plate products should
consistently achieve strengths of about 90 to about 100 ksi with differing
cross-sectional thicknesses.
Table I below shows the composition, density and modulus of an alloy
fabricated into one-inch plate in accordance with this invention. The
sample was solution heat treated at 950.degree. F. and cold water quenched
prior to the aging steps. Table II shows the tensile yield strength
obtained after aging treatments at 250.degree. F. and 300.degree. F. Table
III shows results from fracture toughness measurements with the toughness
measured in accordance with ASTM Standard E561-86, the disclosure of which
is fully incorporated by reference herein.
TABLE I
__________________________________________________________________________
Al--Zn--Mg--Cu--Li Alloy
Measured Composition.sup.1, Density and Modulus
SHT at 950.degree. F. for 2 hours CWQ, Aged 30 hours at 250.degree. F.
Alloy ID
Number
wt. % Zn
wt. % Mg
wt. % Cu
wt. % Li
wt. % Zr
wt. % Fe
wt. % Si
Density.sup.2
Modulus.sup.3
__________________________________________________________________________
606084
9.2 2.16 1.42 1.12 0.11 0.05 0.05 0.099
10.52
__________________________________________________________________________
.sup.1 chemical analyses were conducted by atomic absorption
.sup.2 measured by water displacement
.sup.3 determined in tension
TABLE II
__________________________________________________________________________
Al--Zn--Mg--Cu--Li Alloy
Aging Response at 250.degree. F. and 300.degree. F.
Aged for Aged for Aged for Aged for Aged for
Alloy ID
7 hrs. 15 hrs. 24 hrs. 34 hrs. 50 hrs. Aging
Number
TYS
UTS % E1
TYS
UTS % E1
TYS
UTS % E1
TYS
UTS % E1
TYS
UTS
%
Temp.
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606084
78.1
86.6
8.5 88.0
92.8
5.5 91.7
95.2
3.8 95.2
97.7
3.2 96.9
99.0
3.0 250.degree.
F.
606084
95.7
97.6
2.2 96.6
98.4
1.8 97.4
99.4
2.0 97.4
98.9
1.5 96.3
97.8
1.5 300.degree.
__________________________________________________________________________
F.
Results are the average of duplicate testing in the longitudinal
orientation.
TYS and UTS are in ksi.
TABLE III
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Al--Zn--Mg--Cu--Li Alloy
Strength/Toughness Results and Specific Strength
______________________________________
Alloy Aging TYS (ksi)
##STR1##
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606084 7 hr/25.degree. F.
78.1 32.8
24 hr/250.degree. F.
91.7 23.8
50 hr/250.degree. F.
96.9 21.0
##STR2##
##STR3##
This implies a 22.5% advantage in specific strength over
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7150-T6.
The tensile yield strength and toughness values from Table III are shown
plotted along the x and y axes, respectively, of accompanying FIG. 2.
FIG. 2 shows how the invention achieves greater combinations of these two
main properties. That is, in FIG. 2, strength levels typically increase
from left to right along the horizontal (or x-axis), while toughnesses
increase from the bottom to top of the figure's y-axis, as illustrated.
FIG. 2 shows that strength increases are typically traded for better
toughness properties such that higher strengths may be achieved at lower
toughnesses and vice versa.
In FIG. 3, the specific strength (tensile yield strength divided by
density) of Sample 606084 is shown plotted against the specific strengths
of similar alloy products of 7075-T6 alloy and 7150-T6 alloy.
FIG. 4 shows superimposed sections of iso-specific strength from
experimental alloys containing 1.5 % Cu. This figure is used to select
suitable Al-Zn-Cu-Mg compositions without constituent phases and at a
given level of specific strength.
Specific strength, as used herein, is the tensile yield strength divided by
the density of the alloy. Plate products, for example, made from alloys in
accordance with the invention, have a specific strength of at least
0.75.times.10.sup.6 ksi in.sup.3/ lb and preferably at least
0.80.times.10.sup.6 ksi in.sup.3/ lb. The alloys have the capability of
producing specific strengths as high as 1.00.times.10.sup.6 ksi in.sup.3
/lb.
The wrought product in accordance with the invention can be provided either
in a recrystallized grain structure form or an unrecrystallized grain
structure form, depending on the type of thermomechanical processing used.
When it is desired to have an unrecrystallized grain structure plate
product, the alloy is hot rolled and solution heat treated, as mentioned
earlier. If it is desired to provide a recrystallized plate product, then
the Zr is kept to a very low level, e.g., less than 0.09 wt.%, and the
thermomechanical processing is carried out at rolling temperatures of
about 800.degree. to 850.degree. F. with the solution heat treatment as
noted above. For unrecrystallized grain structure, Zr should be above 0.10
wt.% and the thermomechanical processing is as above except a heat-up rate
of not greater than 5.degree. F/min and preferably less than 1.degree.
F./min is used in solution heat treatment.
If recrystallized sheet is desired having low Zr, e.g., less than 0.1 wt.%,
typically in the range of 0.05 to 0.08 Zr, the ingot is first hot rolled
to slab gauge of about 2 to 5 inches as above. Thereafter, it is reheated
to between 700 to 850.degree. F. then hot rolled to sheet gauge. This is
followed by an anneal at between 500 to 950.degree. F. for 1 to 12 hours.
The material is then cold rolled to provide at least a 25% reduction in
thickness to provide a sheet product. The sheet is then solution heat
treated, quenched stretched and aged as noted earlier. Where the Zr
content is fairly substantial, such as about 0.12 wt.%, a recrystallized
grain structure can be obtained if desired. Here, the ingot is hot rolled
at a temperature in the range of 800.degree. to 1000.degree. F. and then
annealed at a temperature of about 800.degree. to 850.degree. F. for about
4 to 16 hours. Thereafter, it is cold rolled to achieve a reduction of at
least 25% in gauge. The sheet is then solution heat treated at a
temperature in the range of 850 to 1020.degree. F. using heat-up rates of
not slower than about 10.degree. F./min with typical heat-up rates being
as fast as 200.degree. F./min with faster heat-up rates giving finer
recrystallized grain structure. The sheet may then be quenched, stretched
and aged.
The alloy of the present invention is useful also for extrusions and
forgings with improved levels of mechanical properties, for example.
Extrusions and forgings are typically prepared by hot working at
temperatures in the range of 500 to 1000.degree. F., depending to some
extent on the properties and microstructures desired.
While the invention has been described in terms of preferred embodiments,
the claims appended hereto are intended to encompass other embodiments
which fall within the spirit of the invention.
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